In the paper and plastic film industries, a dancer roll is widely used as a buffer between first and second sets of driving rolls in a line of processing machines. The first and second sets of driving rolls define respective first and second nips, which drive a continuous web. The dancer roll, which is positioned between the two sets of driving rolls, is also used in detecting the difference in speed between the first and second sets of driving rolls.
Typically, the basic purpose of a dancer roll is to maintain constant the tension on the continuous web which traverses the respective section of the processing line between the first and second sets of driving rolls, including traversing the dancer roll.
As the web traverses the section of the processing line, passing over the dancer roll, the dancer roll moves up and down in a track, serving two functions related to stabilizing the tension in the web. First, the dancer roll provides a tensioning force to the web. Second, the dancer roll temporarily absorbs the difference in drive speeds between the first and second sets of driving rolls, until such time as the drive speeds can be appropriately coordinated. However, the length of web which the dancer roll can absorb is limited to that length of web which traverses the upward path to the dancer roll and the downward path from the dancer roll.
A web extending between two drive rolls constitutes a web span. The first driving roll moves web mass into the span, and the second driving roll moves web mass out of the span. The quantity of web mass entering a span, per unit time, equals the web's cross-sectional area before it entered the span, times its velocity at the first driving roll. The quantity of web mass exiting a span, per unit time, equals the web's cross-sectional area in the span, times its velocity at the second driving roll. Mass conservation requires that over time, the web mass exiting the span must equal the mass entering the span. Web strain, which is proportional to tension, alters a web's cross-sectional area.
Typically, the dancer roll is suspended on a support system, wherein a generally static force supplied by the support system supports the dancer roll against an opposing force applied by the tension in the web and the weight of the dancer roll. The web tensioning force, created by the dancer system, causes a particular level of strain which produces a particular cross-sectional area in the web. Therefore, the web mass flowing out of the span is established by the second driving roll's velocity and the web tensioning force because the web tensioning force establishes web strain which in turn establishes the web's cross-sectional area. If the mass of web exiting the span is different from the mass of web entering the span, the dancer roll moves to compensate for the mass flow imbalance.
A dancer roll generally operates in the center of its range of travel. A position detector connected to the dancer roll recognizes any changes in dancer roll position, which signals a control system to either speed up or slow down the first and/or second pairs of driving rolls to bring the dancer back to the center of its travel range and reestablish the mass flow balance.
When the dancer roll is stationary, the dancer support system force, the weight of the dancer roll, and the web tension forces are in static equilibrium, and the web tension forces are at their steady state values. Whenever the dancer moves, the web tension forces change from their steady state values. This change in web tension forces supplies the effort that overcomes friction, viscous drag, and inertia, and causes the dancer motion. When the dancer moves very slowly, viscous drag and inertia forces are low and therefore the change in web tension is slight. However, during abrupt changes in mass flow, as during a machine speed ramp-up or ramp-down, the viscous drag, and inertia forces may be several times the web's steady state tension values.
The dancer roll's advantages are that it provides a web storage buffer which allows time to coordinate the speed of machine drives, and the dancer provides a relatively constant web tension force during steady state operation, or periods of gradual change. A limitation of dancer rolls, as conventionally used, is that under more dynamic circumstances, the dancer's ability to maintain constant web tension depends upon the dancer system's mass, drag, and friction.
In processing apparatus for processing a such continuous web, it is common practice to employ both a dancer roll, for purposes of tension control, and a festoon, biased to accumulate and temporarily hold a limited length of the continuous web, but a length substantially greater than the capacity of a dancer roll. The accumulated limited length of web is then played out, or an additional length accumulated, when processing of the continuous web is temporarily interrupted. Such temporary interruption can be, for example and without limitation, change and splicing of a feed/supply roll, or change and splicing of a wind-up roll. Other temporary interruptions can also be accommodated by using the festoon as an accumulator while maintaining operation of various steps in the web manufacture without having to shut the line down.
Such festoon is, by design, a low mass, low inertia device, and is typically biased so as to hold, at steady state operation, an accumulation of web material equivalent to approximately half its capacity for web accumulation. Thus, starting from steady state, the festoon can either accumulate more web if a downstream function is temporarily interrupted or can play out the accumulated length of web if an upstream function is temporarily interrupted. Critical to a festoon is its low mass, low inertia, design.
It is known to provide an active drive to the dancer roll, though such active drive is not known for a festoon, in order to improve performance over that of a static system, wherein the web is held under tension, but is not moving along the length of the web, whereby the dynamic disturbances, and the natural resonance frequencies of the dancer roll and the web are not accounted for, and whereby the resulting oscillations of the dancer roll can become unstable. Kuribayashi et al, “An Active Dancer Roller System for Tension Control of Wire and Sheet.” University of Osaka Prefecture, Osaka, Japan, 1984.
More information about tension disturbances and response times is set forth in U.S. Pat. No. 5,659,229 issued Aug. 19, 1997, which is hereby incorporated by reference in its entirety. U.S. Pat. No. 5,659,229, however, controls the velocity of the dancer roll and does not directly control the acceleration of the dancer roll.
Thus, it is not known to provide an active dancer roll or an active festoon in a dynamic system wherein dynamic variations in operating parameters are used to calculate variable active drive force components for applying active and variable acceleration to the dancer roll or festoon, and wherein appropriate gain constants are used to affect response time without allowing the system to become unstable. Namely, it is not known to drive a dancer roll or festoon so as to nullify physical affects of actual mass and inertia of the dancer roll or festoon. Indeed, no variable drive parameter is known for a festoon.
In a typical structure, a festoon is arranged with a downwardly-disposed array of lower fixed rolls and an upwardly-disposed array of upper movably mounted rolls. The upper rolls are ganged together by a coupling so as to move up and down as a unit when accumulating web material or playing out an accumulated length of web material. In such typical structure, the weight of the respective upper rolls plus coupling apparatus is necessarily considered in designing a dynamic drive system suitable for applying active and variable acceleration to the upper array of festoon rolls. It is not known, however, to apply such dynamic active and variable acceleration to a movable array of festoon rolls where the array of movable festoon rolls is below or beside the cooperating array of fixed-position festoon rolls.